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Degenerative lumbosacral stenosis in dogs

You may not readily recognize degenerative lumbosacral disease in your large-breed patients because they commonly have other concurrent orthopedic diseases. Here's how to identify affected dogs and help them with the right therapy.

Degenerative lumbosacral stenosis is a common cause of cauda equina syndrome and a relatively frequent neurologic disorder in older dogs. If this condition is recognized early, treatment may help alleviate significant morbidity.

The terminology applied to clinical syndromes that affect the spinal cord segments, the spinal nerve roots, and the spinal nerves that contribute to the sciatic nerve innervation of the perineum, urinary bladder, urethral and anal sphincters, and tail can be confusing. Specifically, diseases that affect these structures can result in varying degrees of pelvic limb paresis and urinary and fecal incontinence because they are components of the seventh lumbar (L7) spinal nerve, the first through the third sacral (S1-S3) spinal nerves, and the caudal nerves.

The term lumbosacral disease encompasses any pathologic condition that affects the region of the spinal cord segments that contribute to the sciatic nerve and sacral and caudal nerves. Cauda equina syndrome implies pathologic conditions that affect the last several pairs of spinal nerve roots. Thus, lumbosacral disease and cauda equina syndrome are not specific conditions. Instead, they are the clinical manifestations of a variety of diseases that result in dysfunction of either the lumbosacral spinal cord segments or spinal nerve roots, respectively. Consequently, these two syndromes can result in similar clinical signs.

Causes of lumbosacral disease or cauda equina syndrome include degenerative lumbosacral stenosis; congenital lumbosacral stenosis; acute intervertebral disk extrusion; diskospondylitis; traumatic fractures, luxation, or subluxation; primary neoplasia (involving the vertebral body such as osteosarcoma, hemangiosarcoma, and fibrosarcoma or neural structures such as peripheral nerve sheath tumors or meningioma); metastatic neoplasia such as prostatic carcinoma; inflammatory disease such as neuritis of the cauda equina; spinal empyema; and congenital lesions of the vertebrae.1-3 While not specifically included under the terms lumbosacral disease or cauda equina syndrome, other conditions that can cause similar clinical signs are fibrocartilaginous embolic myelopathy, myopathies, myasthenia gravis, thrombosis of the distal aorta and iliac arteries, and orthopedic disease of the coxofemoral and stifle joints, such as hip dysplasia, cruciate ligament disease, and polyarthropathies.

A detailed discussion of all these causes is beyond the scope of this article. Instead, this article concentrates on the anatomical structures, clinical signs, diagnostic evaluation, therapeutic interventions, and prognosis involved in degenerative lumbosacral stenosis.

ANATOMICAL STRUCTURES

To understand the neurologic signs and various diseases that affect the lumbosacral region of the nervous system, a strong knowledge of the anatomy is needed.

Neural structures

The L4-S1 spinal cord segments are known as the lumbosacral intumescence since the spinal cord is grossly enlarged in this area because of the large collection of neuronal cell bodies in the gray matter that contribute to pelvic limb innervation. As the spinal cord ends, it tapers. This anatomical tapering is called the conus medullaris.4 In dogs, the spinal cord terminates within the vertebral canal at about the level of the L6 vertebra, depending on the dog's size.4 In large dogs, the end of the spinal cord tends to be more cranial than in smaller breeds.5 Consequently, the cell bodies that contribute to the L7 spinal nerve are approximately located within the spinal canal at L4-L5 vertebrae.6 The cell bodies for the sacral and caudal nerve roots are located slightly more caudally. This disparity between the end of the spinal cord and the vertebral canal is the result of the differential growth patterns of these two structures, with the spinal cord ceasing development before the completion of vertebral column growth.4

Even though the spinal cord ends, the dura mater continues caudally as a dural sac, known as the lumbar cistern.4 In dogs, the dural sac continues caudally for a variable distance, occasionally within the sacral vertebral canal. The ventral (motor) and dorsal (sensory) nerve roots exit and enter the spinal cord, respectively, within the dura mater as intradural nerve roots. Although they are known as extradural spinal nerve roots, they remain covered by meninges as they traverse the spinal canal to exit their respective intervertebral foramina.